feat(c3): ADR-073-C C3 — WebSocket 飛輪即時推送

後端：
- stats.py 新增 @router.websocket('/flywheel/ws')
- 每 10 秒推送 flywheel_summary JSON

前端 FlywheelKPICard：
- WebSocket 優先，WS 斷線自動降級到 30s HTTP 輪詢
- onopen 時停止 HTTP polling，onclose 時恢復

2026-04-12 ogt (ADR-073-C C3)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/api/v1/stats.py

@@ -19,9 +19,11 @@
 # @see feedback_lewooogo_modular_enforcement.md
 # =============================================================================
 
+import asyncio
+import json
 from typing import Annotated, Any
 
-from fastapi import APIRouter, Depends, Query
+from fastapi import APIRouter, Depends, Query, WebSocket, WebSocketDisconnect
 from fastapi.responses import PlainTextResponse
 from pydantic import BaseModel, Field
 
@@ -552,3 +554,35 @@ async def get_flywheel_prometheus_metrics(svc: FlywheelStatsDep) -> PlainTextRes
         content=metrics.to_prometheus_lines(),
         media_type="text/plain; version=0.0.4; charset=utf-8",
     )
+
+
+# =============================================================================
+# ADR-073-C C3: WebSocket 即時飛輪推送
+# =============================================================================
+
+@router.websocket("/flywheel/ws")
+async def flywheel_websocket(websocket: WebSocket) -> None:
+    """
+    WebSocket 即時飛輪健康度推送 — ADR-073-C C3
+
+    每 10 秒推送一次 FlywheelSummary JSON。
+    前端連線路徑：ws(s)://<host>/api/v1/stats/flywheel/ws
+
+    Protocol:
+      Server → Client: {"type": "flywheel_summary", "data": {...}, "ts": "ISO8601"}
+      Client → Server: (ignored)
+    """
+    svc = get_flywheel_stats_service()
+    await websocket.accept()
+    try:
+        while True:
+            metrics = await svc.compute()
+            payload = json.dumps({
+                "type": "flywheel_summary",
+                "data": metrics.to_summary_api_dict(),
+                "ts": metrics.computed_at.isoformat(),
+            })
+            await websocket.send_text(payload)
+            await asyncio.sleep(10)
+    except WebSocketDisconnect:
+        pass

apps/web/src/components/dashboard/flywheel-kpi-card.tsx

@@ -1,17 +1,20 @@
 'use client'
 
 /**
- * FlywheelKPICard — ADR-073-C C2
+ * FlywheelKPICard — ADR-073-C C2 + C3
  *
- * 飛輪健康度 KPI 面板，消費 GET /api/v1/stats/summary。
- * 30 秒輪詢，無快取假數據。
+ * 飛輪健康度 KPI 面板。
+ * C2: 初始載入 GET /api/v1/stats/summary（HTTP fallback）
+ * C3: WebSocket /api/v1/stats/flywheel/ws 即時推送（10s 更新）
  *
- * 2026-04-12 ogt (ADR-073-C C2)
+ * 2026-04-12 ogt (ADR-073-C C2 + C3)
  */
 
-import { useEffect, useState } from 'react'
+import { useEffect, useRef, useState } from 'react'
 
 const API_BASE = process.env.NEXT_PUBLIC_API_URL ?? ''
+// ws(s):// mirror of NEXT_PUBLIC_API_URL
+const WS_BASE = API_BASE.replace(/^https/, 'wss').replace(/^http/, 'ws')
 
 interface FlywheelSummary {
   playbook_count: number
@@ -28,9 +31,12 @@ interface FlywheelSummary {
 export function FlywheelKPICard() {
   const [data, setData] = useState<FlywheelSummary | null>(null)
   const [error, setError] = useState(false)
+  const wsRef = useRef<WebSocket | null>(null)
 
+  // C2: HTTP fallback (initial load + 30s poll when WS unavailable)
   useEffect(() => {
     let cancelled = false
+    let pollId: ReturnType<typeof setInterval> | null = null
 
     const load = () => {
       fetch(`${API_BASE}/api/v1/stats/summary`)
@@ -40,8 +46,44 @@ export function FlywheelKPICard() {
     }
 
     load()
-    const id = setInterval(load, 30_000)
-    return () => { cancelled = true; clearInterval(id) }
+
+    // C3: WebSocket — upgrades from polling when available
+    const connectWS = () => {
+      if (!WS_BASE) return
+      const ws = new WebSocket(`${WS_BASE}/api/v1/stats/flywheel/ws`)
+      wsRef.current = ws
+
+      ws.onopen = () => {
+        // WS connected — stop HTTP polling
+        if (pollId) { clearInterval(pollId); pollId = null }
+      }
+      ws.onmessage = (e) => {
+        try {
+          const msg = JSON.parse(e.data)
+          if (msg.type === 'flywheel_summary' && !cancelled) {
+            setData(msg.data)
+            setError(false)
+          }
+        } catch { /* ignore malformed */ }
+      }
+      ws.onclose = () => {
+        // WS closed — fall back to polling
+        if (!cancelled && !pollId) {
+          pollId = setInterval(load, 30_000)
+        }
+      }
+      ws.onerror = () => ws.close()
+    }
+
+    connectWS()
+    // Also start polling as backup until WS opens
+    pollId = setInterval(load, 30_000)
+
+    return () => {
+      cancelled = true
+      if (pollId) clearInterval(pollId)
+      wsRef.current?.close()
+    }
   }, [])
 
   const fmt = (n: number | undefined, digits = 0) =>